A conventional motor control apparatus controls a motor, which has phase windings of a plurality of phases. This motor control apparatus controls rotation of a motor by controlling on/off of high-side FETs (field effect transistors) and low-side FETs in a plurality of phases of an inverter circuit. Safety design is implemented in both aspects of hardware and software so that a failure, in which both a high-side FET and a low-side FET of a predetermined phase are short-circuited at the same time, does not arise. This failure is referred to as an arm short-circuit failure of a predetermined phase. If safety design against the arm short-circuit failure is not implemented, large current continues to flow to the inverter circuit. It is likely that this will cause burn-out of the inverter circuit in the end.
According to a motor control apparatus for an electric power steering system of a vehicle disclosed in the following patent document 1, rotation of the motor is controlled by controlling on/off state of high-side FETs and low-side FETs in three phases of an inverter circuit. In this motor control apparatus, a shunt resistor is provided between each of the low-side FETs and the ground. Current flowing in the shunt resistor is detected by detecting a voltage difference across the shunt resistor, amplifying the voltage difference and converting the amplified voltage difference to current. A current value acquired when all the low-side FETs of all three phases are turned on is set as a first current value. A current value acquired when all the low-side FETs of all three phases are turned off is set as a second current value. The first current value is corrected by the second current value, which is used as an offset correction value. Short-circuit failure in the high-side FET and the low-side FET is detected by comparing a third current value, which is the corrected first current value, with a threshold value. According to a motor control apparatus for an electric power steering system disclosed in the following patent document 2, short-circuit of either FET is detected by comparing a second current value, which is detected when low-side FETs of all three phases are turned off, with a threshold value.
Patent document 1: JP 2003-324985A
Patent document 2: JP 2003-324928A (US 2004/0027083 A1)
Even if safety design is exercised in an inverter circuit, it is still likely that unexpected secondary failure will be caused as a chain reaction by one other failure or an arm short-circuit failure will be caused by an uncontrollable operation of a CPU or the like. According to the method disclosed in patent document 1, when either the high-side FET or the low-side FET in three phases is short-circuited, the third current value exceeds the predetermined value. It is thus possible to determine short-circuit abnormality. However, if both the high-side FET and the low-side FET of the same phase are short-circuited at the same time, the third current value, which is corrected, does not exceed the threshold value. It is therefore likely that the arm short-circuit failure cannot be detected.
According to the method disclosed in the patent document 2, whether one of or both of the high-side FET and the low-side FET of the same phase is short-circuited, the same result is detected. For this reason, it is not possible to differentiate the short-circuit failure of the high-side FET, the short-circuit failure of the low-side FET and the arm short-circuit failure, in which both the high-side FET and the low-side FET of the same phase are short-circuited. It is not possible to shorten detection time only for the arm short-circuit failure.
If the arm short-circuit failure arises, battery voltage falls. It is therefore possible to detect the arm short-circuit failure by monitoring fall of the power supply voltage of the inverter circuit. Possibility of erroneous detection may rise, because the voltage falls due to a plurality of reasons other than the arm short-circuit failure. The other reasons include fall of voltage due to overload on other electric devices. Even in case of the arm short-circuit failure, voltage falling due to wiring resistance between the battery and an electronic control unit (ECU) changes. Further, it takes time for a central processing unit (CPU) to recognize fall of the voltage, because the voltage actually falls only after the current is drawn from an aluminum electrolytic capacitor provided between the inverter circuit and the battery and a low-pass filter (LPF) is provided as hardware for analog/digital (A/D) conversion of voltage.